Media and Tissue Testing - Media Testing Methods

It is necessary to periodically check the pH, electrical conductivity and individual nutrients in a growing medium to monitor crop status.

To do so, collect a two-cup sample of growing medium taken from several plants of the same cultivar and age and mail it to a horticulture testing laboratory. Most labs require 2-3 business days to process the samples. However, some will offer the option of expediting sample analysis in one day for an extra charge. Remember the time it takes to mail the samples to the lab must be added to the processing time to know when the results will be available.

Many growers want to know this information as soon as possible in order to make critical decisions about fertilizer application, leaching or finding corrective measures if the pH is out of desirable range. If you have access to pH and E.C. meters, then these parameters can be tested in-house, at your location. There are a few meters that do test for an individual nutrient, but collectively they can be expensive. Often, in-house testing is generally for testing pH and E.C., which will catch most of the common fertility problems (pH too high or too low, fertility rates too high or too low).

To prepare a growing medium sample for in-house testing for pH and E.C., the growing medium sample must be mixed with a specific volume of deionized water and allowed to sit for at least a half hour to a full hour. From this sample, the pH and E.C. can be measured. There are several different methods used to prepare growing medium samples for testing. Keep in mind that the results from one test method, such as E.C., cannot be interpreted using normal ranges from another test method. So whatever the sampling procedure, it is important to use the normal ranges from that test method and not from another. The test methods, how to prepare the samples and the pros and cons of each are discussed below. Keep in mind that these procedures work for both unused growing medium and in-use growing medium from a crop.

Saturated Media Extract (S.M.E.)

The saturated media extract involves taking growing medium, placing it inside a cup and then adding enough deionized water to saturate the sample. Samples must be taken from the lower portion of plant root balls and any control release fertilizer prills must be removed from each sample. Deionized water should be mixed into the growing medium until the growing medium's surface glistens or looks shiny. There should be no water puddled on the surface of the growing medium sample, nor should the growing medium be “floating” in water.

An advantage of the S.M.E is that you do not need to consider the starting moisture content of the growing medium as with the other procedures. For example, if the sample is wet due to a recent irrigation, then less deionized water is required to saturate the growing medium sample compared to a dry sample. Another advantage is that most horticulture laboratories use this same procedure, so your results can be directly compared with laboratory results.

The disadvantage to this procedure is that it is slightly destructive, meaning growing medium must be taken from several plants. This can take some time, especially when blending the sample together to make it uniform. If controlled-release fertilizer prills are present in the growing medium, they should be first removed so they are not damaged when preparing the sample.

2:1, 5:1 (Ratio of Water: Growing Media) methods

Like the S.M.E. analysis, the growing medium is placed in a cup and deionized water is added. In this case, either two parts deionized water is added to one part growing medium (2:1 method) or five parts deionized water is added to one part growing medium (5:1 method). Ideally, the growing medium should start with moderate moisture content and not be saturated or dry. It is best to sample plants at the same time relative to an irrigation event to reduce variability in moisture content of the starting sample.

The advantages of this method is that everything is prepared as simple volume measurements, there is no guessing as to what glistening is and you do not have to worry about adding too much water, which could happen with S.M.E. There may be a few horticultural testing laboratories that use the 2:1 method, so it is easier for comparison, but most use S.M.E.

The disadvantages are the same as S.M.E., in that it requires the growing medium to be taken from several containers. Although knowing how much water to add is easy, the initial moisture content of the sample has some influence on the E.C. results.

Pour-Through and Squeeze Methods

These methods are non-destructive, meaning no growing medium is removed from containers, and involve extracting water from the growing medium within a container or cell. The pour-through method is used for cell packs and larger pots while the squeeze method is for plugs or liners smaller than a 50-cell plug tray. Both methods require saturating the growing medium one hour before testing. If you constant feed, use the fertilizer solution to thoroughly water the plants, and if you pulse feed (periodic feeding), use clear water. 

For the pour-through method, pour deionized water over the growing medium's surface, as listed in the table below, and collect the leachate that flows out of the bottom of the container. The goal is to obtain 50 ml (2.5 ounces) of solution per sample. Do not exceed 60 ml (3.0 ounces) as the sample will be too diluted, resulting in lower E.C. values.

For the squeeze method, collect the solution from the plug by either pressing down on the growing medium's surface or removing the plug and squeezing the solution out of the growing medium. The volume of solution needed depends on how much is required to operate the pH and E.C. meters.

The advantage of these methods is that they are non-destructive and involve extracting water from a container or cell. They are quick and easy. The pour-through method is preferred if controlled-release fertilizer has been incorporated into the growing medium, since it does not break the prills, which can happen with the above procedures.

Standards for both methods are not as well-established. Care must be used to collect the correct amount of leachate as too much will dilute the salts, reducing E.C., and collecting too little leachate will elevate the E.C. Do not use these methods for subirrigated crops as excessive surface salts will be leached down through the growing medium profile into the collected solution and affect E.C. and pH results.

In the the article, Media and Tissue Testing - Part 3, we will discuss how to measure pH and E.C. from the various test methods described in this article. Then the results will be contrasted from one method to another to see how each method varies in their normal ranges for E.C.